I. Field of the Invention
The present invention relates to the fields of biology and diagnostics. More specifically, the invention deals with “drop”-based technologies that permit simple, fast and accurate detection of most any analyte in environmental, plant, medical and other samples.
II. Related Art
The engineering of diagnostic devices suitable for the low resource, point-of-care (POC) setting is challenged by design criteria that include low cost, simplicity of operation, and minimal reliance on external instrumentation (Yager et al., 2008 and Yager et al., 2006). The ideal biosensor requires no on-board power source and produces an easily detectable signal in a short period of time. Harnessing the hydrodynamics of an evaporating drop represents one possible means of satisfying these design requirements. Fluid motion inside an evaporating sessile drop occurs spontaneously due to a non-uniform evaporation rate along the surface of the drop, which produces predictable hydrodynamic properties (Deegan, 2000, Deegan et al., 2000 and Deegan et al., 1997). The resulting flow fields include the primary radial flow and secondary flows caused by direct and indirect effects of the non-uniform evaporation rate, respectively. (Deegan, 2000, Deegan et al., 2000, Deegan et al., 1997, Barash et al., 2009, Hu & Larson, 2005 and Hu & Larson, 2006).
The primary radial flow field is the most commonly and easily observed flow pattern and is what causes a ring to form in an evaporating coffee drop, known as the “coffee ring effect” (Deegan et al., 1997). The non-uniform evaporation rate across the surface of the drop induces a radial current that carries material in solution to the periphery of the drop resulting in a concentrated ring pattern (Deegan, 2000, Deegan et al., 2000, Deegan et al., 1997). A precondition for this hydrodynamic property is the presence of colloidal particles that pin the contact line preventing it from receding during evaporation (Deegan, 2000, Deegan et al., 2000, Sangani et al., 2009). Fluid flows in a radial direction to replenish solution preferentially lost at the edge where solvent molecules evaporate at the greatest rate. This naturally-occurring phenomenon is easily observed under a microscope with which the two-dimensional radial motion of micron-sized particles can be resolved. The physical basis and flow characteristics of this mass transport system have been previously described (Hu & Larson, 2005, Hu & Larson, 2005, Trantum et al., 2013, Hu & Larson 2005 and Harris & Widjaja, 2008).
Recently, several groups have reported using the primary radial flow to discern information about the components of the solution. Wong et al. demonstrated that the size exclusion geometry of the contact line in an evaporating drop can be used for chromatographic separation of colloidal particles (Wong et al., 2011). Several groups have shown that dried patterns of drops of biological fluids can be used to characterize sample components and potentially be used as an indicator of disease (Tarasevich & Pravoslavnova 2007a, Tarasevich & Pravoslavnova 2007b and Brutin et al., 2011).
The inventors recently reported a diagnostic assay in which the primary radial flow in an evaporating water drop organizes functionalized magnetic particles to generate a colorimetric response based on the presence of a biomarker (Trantum et al., 2012). This proof-of-concept assay successfully detected a peptide mimic of the malaria biomarker protein, Plasmodium falciparum histidine rich protein (pJHRP-II), using a Ni(II)NTA biorecognition element conjugated to the surface of the particles. Evaluation of this assay design reveals several limitations. First, the assay requires precise alignment of the drop over a magnetic field, operationally tedious for what is intended to be a simple and rapid assay. Second, the limit of detection, approximately 200 nM, must be at least 1000× more sensitive for clinical relevance in malaria detection. Finally, the assay does not work in the presence of salt at physiologic levels. Salt crystal formation at the end of the evaporation process significantly alters particle deposition patterns resulting in false results. Thus, even this improved approach would benefit from further improvements.